Field of the Invention
One or more embodiments of the invention are related to the field of sensors for contact detection or for collision detection, for example between a robot bumper and the environment. More particularly, but not by way of limitation, one or more embodiments of the invention enable a compliant touch sensor.
Description of the Related Art
Collisions between moving objects are inevitable and are sometimes desired. In most situations it is desirable to minimize damage caused by collisions. The best ways to minimize damage are to detect a collision and implement corrective action as soon as possible, and to use bumpers or other features to absorb the impact. Sensors for collision detection are of particular importance for applications such as robotics where computers control or may override human control of the motion and can react almost instantaneously upon the sensing of contact. Collision sensors may also be used in non-robotic applications including, but not limited to human controlled vehicles or appliances when there is a computer override in the case of a collision, or simply to provide early feedback to the operator.
The need to detect collisions and minimize the resulting damage has long been recognized. Several solutions are known in the art.
Some bumpers have mechanical switches that can detect motion or deformation. Designs for these bumpers involve tradeoffs between stiffness, mechanical compliance, sensitivity and coverage. In general, larger bumpers with significant coverage are inherently stiffer and less sensitive. Large bumpers are required for robots operating in typical home or office environments, since robots may encounter obstacles at arbitrary heights. For example, on most robot vacuum cleaners the entire front surface is a hard spring loaded bumper. The large size results in a relatively stiff bumper. Also, depending on the mechanical design it may be hard to sense small areas in order to provide feedback for where the collision occurred. Corners are also a problem for mechanical bumpers but are critical in practice because obstacles are often struck when a moving system tries to pass closely beside an object.
Strain gages, pressure sensitive sheets, and similar systems are often very sensitive, but typically have very little compliance. Such systems are employed on robot grippers and give feedback for how much pressure these mechanical hands are applying to an object. However, the low compliance of these systems makes them unsuitable for many applications. Resistive solutions are known to exist that provide variable resistance sensors for measuring forces and minimizing displacement.
Forward-looking non-contact sensors such as ultrasonic, LASER, time-of-flight and LIDAR have significant compliance (distance traveled after detection before damage) because they are non-contact and detect obstacles at a distance. However, these systems are not 100% reliable; therefore mobile robots that include these sensors almost always include backup mechanical sensor/bumper combinations. These systems perform very poorly or not at all when sensing objects that are close. This presents problems for systems that are intended to operate in tight areas, such as a robot vacuum cleaner that is intended to get as close to walls and obstacles as possible, or robot arms that are to work side-by-side with people. Higher quality non-contact sensors are also typically expensive.
Sensor such as laser line breaks and laser curtains have a transmitter and receiver that detect when an object blocks the line of sight between the two. These systems can work well for stationary obstacles such as walls or doorways, but they do not provide a general-purpose collision detection solution.
In summary, each of the contact sensing solutions known in the art involve tradeoffs between coverage, reliability, and compliance. There are no known contact sensing solutions that provide very high reliability, high compliance for a soft impact, fast reaction to sense contacts quickly when they occur, and coverage of a large surface area.
For at least the limitations described above there is a need for a compliant touch sensor.